1. Field of the Invention
The present invention relates to a system for treatment of an aqueous medium containing (meth)acrylic monomer, as for example wastewater produced by rinse removal from impregnated porous parts of excess amounts of a (meth)acrylic monomer-based impregnant composition which is curable under anaerobic conditions (i.e., in the substantial absence of oxygen) by free radical polymerization in the presence of a suitable initiator.
2. Description of the Related Art
Impregnation sealing of porosity in porous parts frequently is carried out by introducing sealant compositions into the porosity under a pressure differential, by wet vacuum and dry vacuum techniques which are well known in the art.
Impregnation sealing of porosity may also be carried out by wicking, wherein the impregnation sealant is flowed across the surface of the porous part and allowed to wick into the voids thereof, during a selective period of time. Once the wicking action is completed, the impregnated parts are washed to remove excess surface impregnant. With certain parts, wicking may be enhanced by creating a vacuum inside a cavity within the part. For example, with large parts such as engine blocks, which may not be able to be accommodated by conventional wet vacuum and dry vacuum systems, all but one of the ports or openings into the interior of the engine block is closed off, followed by attachment of a vacuum pump to the remaining port. After the vacuum pump is actuated, air will attempt to enter the engine block through the voids or pores in the porous metal constituting the engine block. The resulting vacuum and air flow will carry any resins which are applied to the surface of the metal part into the porosity thereof.
The sealant compositions typically employed in the aforementioned impregnation applications include a wide variety of self-curing anaerobic sealants, which are curable via freeradical polymerization in the presence of suitable free-radical initiators, e.g., peroxy-type initiators, as well as thermal- [curing sealants, and sealants which cure by both anaerobic and heat cure mechanisms.
Illustrative of the (meth)acrylic monomer-based anaerobic impregnant compositions known in the art are the compositions described in U.S. Pat. Nos. 3,672,942; 3,969,552; Reissue 32,240; and 4,632,945.
Once the impregnant composition is introduced into the porosity of the parts to be sealed, the parts are transferred to an agitated water rinse zone for removal of any remaining surface accumulations of sealant or extraneous sealant which is trapped in blind holes of the impregnated parts. After removal of the excess sealant in the agitated water rinse zone, the impregnated parts are passed to a tank containing a catalyst activator solution which serves to cure the sealant composition at the entrance of the porosity. This creates a hardened plug or cap in the outer portion of the pores, trapping the remaining resin for anaerobic self-cure.
In order to maximize excess sealant removal action in the rinse step, clean make-up water is introduced to the rinse tank either continuously or in batch fashion, with corresponding discharge of monomer-containing aqueous medium from such tank as wastewater effluent. The agitated water rinse washing of the impregnated parts to remove excess sealant material therefrom results in the sealant passing into the aqueous washing medium. The impregnant compositions containing (meth)acrylic monomer are substantially insoluble in the aqueous washing medium, and thus form a dispersion or emulsion of the (meth)acrylic monomer in the wash water.
In conventional impregnation systems utilizing (meth)acrylic monomer-based sealant compositions, the monomer content of the wastewater discharged from the aqueous rinse step may be on the order of from about 0.01 percent to about 8-10 percent by weight (based on the weight of wastewater), or even higher, with concentrations of 0.1-3 percent typically being encountered.
Current legislative and environmental considerations dictate that the wastewater be treated to remove its monomer content. It is desirable for any such treatment of the wastewater to be consistent and effective. Biological treatment, while widely employed for wastewater purification, has the disadvantage that changes in environmental conditions, particularly temperature, may adversely affect the population dynamics and stability of the biomass employed in the wastewater treatment process, thereby lowering the biological activity of the microorganisms effecting treatment. Further, the formulation of the impregnant composition may change with time, and a biomass or microbial population which is efficacious in the treatment of one composition may be deficient or even useless for treatment of wastewater having a different composition.
Prior copending application Ser. No. 07/526,429 of C. M. Muisener and E. K. Welch filed May 18, 1990 for "POROUS ARTICLE IMPREGNATION RESIN COMPOSITION, AND SYSTEM FOR TREATING IMPREGNATION PROCESS WASTE WATER," describes a method of treating the wastewater produced by aqueous washing of impregnated porous articles to remove excess impregnant therefrom. In the method disclosed in this application, the impregnant comprises a curable composition containing (meth)acrylate monomer and initiator species, including a first initiator for heat-cure and/or anaerobic cure (polymerization) of the (meth) acrylate monomer, and a second, actinic radiation photoinitiator for photopolymerization of the monomer. The impregnant also contains an agglomerating agent which is agglomeratingly effective for the (meth)acrylic monomer and/or corresponding polymer in aqueous medium. In this wastewater treatment process, at least a portion of the monomer-containing wastewater is exposed to actinic radiation polymerizingly effective for the monomer in the presence of the photoinitiator, to form corresponding polymer from the monomer; the agglomerating agent facilitates the aggregation of the monomer and/or resulting polymer, to enhance the efficacy of the process. By such treatment, the polymer is susceptible to recovery from the effluent by conventional physical separation methods, such as filtration.
In the practice of the wastewater treatment method in the above-described application Ser. No. 07/526,429, the actinic radiation employed for polymerization of the monomer in the wastewater typically has a very limited depth, e.g., about two inches or less. Accordingly, the effluent liquid must be treated in a thin film, so that the source of actinic radiation provides effective irradiation of the entire mass of the wastewater undergoing treatment. Moreover, any pigments, colorants, fillers, etc., which introduce turbidity, opacity, or otherwise attenuate the actinic radiation transmissivity of the wastewater can severely adversely affect the treatment efficiency, and result in an effluent which does not meet legislative or regulatory constraints, or which is otherwise unsuitable for discharge to final receiving waters.
U.S. Pat. No. 4,130,674 issued Dec. 19, 1978 to W. J. Roberts et al describes the treatment of water used to trap oversprayed coating materials in spray booths which are utilized in the spray application of paints, lacquers, and organic finishes. Such spray booths use water curtains, which collect in a bottom water reservoir for discharge; raw coating material accumulates in such installations, resulting in blockage of pipes and pumps which circulate the water. The patent describes a composition which is added to the spray booth water, comprising a mixture of (i) a long carbon chain surface-active nitrogen derivative containing at least one C.sub.8 hydrocarbon chain, and at least one member selected from the class consisting of (ii) a polyvalent water-soluble or dispersible metal salt, and (iii) a nonionic or anionic high molecular weight polymer. The foregoing components are employed as a solution which is added to the spray booth water so that such water contains from about 0.05 percent to about 2 percent of the solution. The patent describes pH adjustment of the spray booth water. Such treatment is said to convert a wide variety of paints to an "ecologically harmless sand-like waste" (column 3, line 40) so that clear water is provided for recycling in the spray booth system.
U.S. Pat. No. 2,029,961 issued Feb. 4, 1936 to O. M. Urbain describes a method of removing carboxylic acids of the formula: ##STR1## from water in which such carboxylic acids are in solution. The disclosed process entails optional pH adjustment of the solution to below pH 7, following which an acidic amine salt reagent is added to the solution and agitated for a brief period. Suitable acidic amine salt reagents include salts of amino acids, i.e., salts of the acid and/or amine parts of the molecule. The acidic amine salt reagent reacts with the carboxylic acid to form addition compounds "that are either unobjectionable or insoluble and removable by sedimentation" (column 1, lines 10-11).
U.S. Pat. No. 4,530,768 issued Jul. 23, 1985 to K. Tanihara et al describes the treatment of wastewater containing iron-cyanide complexes including ferricyanides, in which the wastewater is treated with a combination of a sulfite and a thiosulfate as a reducing agent. This effects reduction of the ferricyanide ions and the precipitation of ferrocyanide ions in the form of zinc ferrocyanide. The combination of a thiosulfate with a sulfite reducing agent is said to synergistically enhance the reducing activity of the composition. In its background section, this patent references Japanese Patent Publication Number 57-5598, which teaches reducing ferricyanide compounds in wastewater, in the presence of zinc ions, using reducing agents such as thiosulfates, sulfites, pyrosulfites, dithionites, salts of hydrazine, salts of hydroxylamine, and the like (column 2, lines 5-8).
U.S. Pat. No. 4,326,912 issued Apr. 27, 1982 to C. C. Mollett describes the removal of contaminants such as adhesives or ink particles from wastepaper pulp, in which a monomer is dispersed in the pulp to form beads or droplets which take up the impurity species, following which the monomer is polymerized and the resulting polymeric particles are physically separated, such as by screening. Prior to the polymerization, oxygen preferably is removed from the monomer phase, as for example by nitrogen displacement, so that polymerization is not later inhibited. The ratio of monomer to liquid (e.g., water) may be in the range of 1:100 to 1:200 by volume. The initiator is added in an amount which may be on the order of about 1 percent by weight of the monomer. Initiators for the polymerization reaction are described as including organic peroxides and hydroperoxides, and azoisobisbutyronitrile. Useful accelerators include those based on substituted amines and anilines.
U.S. Pat. No. 4,851,130 issued Jul. 25, 1989 to R. D. May, describes the removal of oxygen from an aqueous system by erythorbic acid, erythorbate, ascorbic acid, or ascorbate, with the aid of activated carbon. The system is said to be useful in aqueous systems such as boiler feed water and oil well injection water. In its background section, this patent references Meyers et al U.S. Pat. No. 4,627,921 as disclosing the deoxygenation of water at ambient temperature by passing the water with a reducing agent through an adsorbent, absorbent bed which may be made of activated carbon. The reducing agent may be hydrazine, morpholine, sodium sulfite, or sodium acid sulfite. In the process of the May patent, the oxygen-containing system preferably is contacted with the oxygen scavenger prior to contacting with the activated carbon. The patent describes the oxygen scavenger as preferably being a soluble salt of erythorbic acid such as ammonium, sodium, potassium, calcium or zinc erythorbate, with sodium erythorbate monohydrate being particularly preferred.
U.S. Pat. No. 4,278,582 issued Jul. 14, 1981 to J. R. Miller, describes the removal of acrylonitrile from an aqueous medium. The reactive substance is selected from the group consisting of ammonia, ammonium hydroxide, thioalcohols, hydrogen sulfide and its salts, alkali metal and alkaline earth metal carbonates, phosphates, silicates, and mixtures thereof. The patent discloses that the aqueous reaction medium for polymerization of acrylonitrile may contain co-monomer species which are polymerized with the acrylonitrile, such as alkyl acrylates. The patent describes the acrylonitrile polymerization reaction as involving free radical catalysts such as peroxygen compounds, azo compounds, and the like. Alkali metal and ammonium persulfate catalysts are said to be usefully employed by themselves or in activated redox systems. The reaction of acrylonitrile with the reactive substance is carried out in an agitated system, at a temperature of from about 0.degree. C. to 90.degree. C., with the acrylonitrile reaction being favored by increasing temperature.
Japanese patent application number 50-47237 filed Apr. 17, 1975 and published Oct. 25, 1976, in the name of Takeshi Hyrayana, et al, describes a method for treating eluted waste liquids containing high molecular weight photosensitive resins and low molecular weight reaction monomers. A polymerization initiator is added to the eluted waste liquid, and the liquid is subjected to heat or light to cause the monomers therein to react. The reaction product waste liquid then is passed to a concentration tank, from which it is repeatedly circulated through an ultrafiltration apparatus, until the liquid is concentrated by a factor of 10-15 in the concentration tank. During the ultrafiltration processing of the waste liquid, a filtrate is produced which is reused in the concentration tank until a predetermined concentration in the concentration tank is achieved, at which time the ultrafiltration apparatus is shut down. Thereafter, the concentrated waste liquid is mixed with heavy or light oil, and subjected to combustion treatment.
West German Offenlegungsschrift 27 05 169 published Aug. 10, 1978 in the name of Dr. Hansjochen Schultz - Valtz, discloses a process for treating wastewaters containing about 15-80% by weight of emulsified polymerizable liquid substances. In this process, 0.1-10 weight percent polymerization initiators, based on the total weight of wastewater, are added to the wastewater. The polymerization is carried out at temperatures of between 0.degree. C. and 180.degree. C. and pressures up to 10 bar, followed by separation of the polymerization products from the water. This publication discloses the use of polymerization initiators such as peroxides and/or hydroperoxides, and the use of polymerization accelerators, such as tertiary amines, organic cobalt, and vanadium salts. The process disclosed in this publication is described as being applicable to treatment of wastewater from impregnation of metal castings with unsaturated polyesters. (Meth)acrylic esters are also disclosed as polymerizable emulsified monomers to which the disclosed process is applicable.
U.S. Pat. No. 4,147,821 issued Apr. 3, 1979 to P. D. Young describes a composition for impregnating porous articles such as metal castings. The disclosed impregnant composition includes a monomer system, such as mixtures of monofunctional and polyfunctional (meth)acrylic esters, in combination with a peroxy catalyst and an inhibitor (column 3, lines 1-21). The patent discloses that "[i]t may be convenient to include emulsifying agents (which term includes wetting agents and detergents) in the impregnant as an aid to subsequent water washing." The patent specification refers to effluent disposal problems associated with the use of the impregnant composition. The patentee discloses that when the level of emulsified impregnant has built up to a concentration of 1-5 percent, the wash liquid may be pumped to a separate tank and heated as for example at 100.degree. C. for two minutes to polymerize the composition and precipitate same ("on cooling, the nearly clear supernatant liquid may be drained into the sewer leaving a damp granular solid which, being a polymer, may be disposed of as an inoffensive almost odorless solid waste" ). The patent states that the curdling (polymerization) may take twenty minutes at 90.degree. C.
It is an object of the present invention to provide a treatment process for wastewater, e.g., wash water resulting from rinsing of porous parts impregnated with (meth)acrylic monomer-based sealants, which is adaptable to remove (meth)acrylic monomers therefrom in a simple, effective, and economical manner.
It is a further object of this invention to provide such a wastewater treatment process which works effectively over a range of resin strengths in a readily controllable and reliable manner.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.